Various efforts have been made to develop a method of manufacturing micron-size resin granules with a narrow distribution of grain size and many patents have been filed therefor.
One of such methods is the so-called suspension-polymerization method wherein liquid drops of a vinyl monomer are formed in water in the presence of an appropriate dispersion stabilizer and polymer granules are synthesized using appropriate oil-soluble polymerization initiators.
However, when polymerization is conducted under the ordinary agitating conditions, the polymer sticks to the wall of the reaction vessel, agitating fan, etc. and, when polymer granules are produced, the distribution of grain diameter is mostly governed by the probability factor of the division and joining of liquid drops during polymerization. As a result, only those resins having an extremely wide grain size distribution are obtained. As a countermeasure therefor, there has been proposed a method wherein polymerization conditions are so modified that, for example, the monomer once goes through bulk polymerization or a part of the polymer is dissolved in the monomer and after thus providing some viscosity, suspension polymerization is conducted. Alternatively, polymerization is carried out by using various suspension stabilizers having strong surface activity, or inorganic powder which is difficult to dissolve in water, or combinations of such materials, but the improvement of grain size distribution realized by such methods has been only slight.
On the other hand, according to the seed polymerization swelling method described in Japanese Kokai Publication 58-106554, it is possible to form vinyl polymer granules of extremely narrow grain size distribution and it is also possible to bridge and gelate the monomer using polyfunctional monomers such as divinyl benzene.
However, according to this method, the growth of the grains occurs in multi-stages and therefore the steps are extremely long; thus, its industrialization potential is poor. Besides, it is difficult to uniformly charge foreign matters such as pigments in the granules by such method.
In order to improve such drawbacks, for example, Japanese Kokai Publication 3-200976 proposes a method of forming colored polymer granules by dispersion polymerization in non-aqueous (or solvent/water mixture) solvents. However, since the method uses solvents, problems occur as to the waste liquid treatment and in respect to safety.